This invention generally relates to switching circuitry and in particular concerns debounce logic circuitry for eliminating transient and undesirable pulses generated by mechanical contacts within switches.
With pushbutton switches and keyswitches such as those found on modern keyboards, the problem of contact bounce is well known. Depression of the pushbutton abruptly causes the mechanical contacts within the switch to close and as a result of the mass of the contacts and the instantaneous force to which they are subjected, the contacts tend to bounce generating extraneous closures which may send false signals to a downstream signal processor. Under such conditions, means are advisable for determining whether the switch has been successively actuated for purposes of generating a plurality of repetitive signals or whether the sequence of contact closures was generated by contact bounce.
Prior work in this area includes U.S. Pat. No. 4,028,560 to Bainter which discloses a switch debounce circuit having a serial-to-parallel shift register with a mechanical switch and clock source input. The outputs of the shift register and the switch signal are applied to a NOR gate to obtain the debounced output. Also, U.S. Pat. No. 3,792,466 to Arnold et al teaches a debounce circuit wherein the switch and clock signals are applied to a shift register and the debounced output is obtained from an ANDNOT gate. While each of the patents is suitable for their intended purpose, neither patent combines the useage of a NAND gate and a shift register in such a simple and inexpensive manner while producing distinctive operating characteristics.